Image registration may be achieved with two classes of image pairings, namely those taken explicitly for stereo applications (stereo pairs), and those pairs with common coverage which were not explicitly taken for stereo applications (common coverage pairs). Stereo pairs usually restrict the resolution of the images to similar values, and limit the convergence, asymmetry and bisection angles. The images of a stereo pair, therefore, have a similar appearance, and are preferred for stereo applications because the stereo effect is more readily perceived.
Images in a common coverage pair may be taken on different occasions, possibly months apart, and may appear significantly different due to changes in lighting, season, ground features, and resolution. Additionally, the convergence and asymmetry angles may be extreme. Common coverage pairs can be used for stereo viewing, but the difference in appearance between images, and extreme convergence and asymmetry angles often make the stereo effect difficult to perceive.
Image registration has many applications for both stereo and common coverage pairs. The stereoscopic effect is used in three-dimensional mensuration and mapping, charting and geodesy (MC&G) applications to extract three-dimensional measurements not obtainable from monoscopic images. Image registrations are also used in other MC&G applications, such as terrain elevation extraction, in which automated algorithms perform search strategies in spaces constructed from registered images.
The use of image registration and common coverage pairs also arises in general search applications. In general search applications, two images are registered for comparison as a "reference" and a "mission" image. Image registration is also used for the automatic location of fixed targets. Given the coordinates of fixed target in a "reference" image, the target can be located to within a few pixels automatically in a "mission" image if the two images are registered. In comparison, alternative methods based only on camera pointing information from the "mission" image can give errors of thousands of pixels.
Given a pair of images with common ground coverage, it is extremely difficult to view them in stereo without a registration process. It may be possible to view them simultaneously as monoscopic images, but image manipulations presently available rarely enable stereo perception. Even when stereo perception is possible, any measurement made from such a set-up will be inaccurate, the images will have to be readjusted if the field of view is moved, and the stereo effect will only be perceptible for short periods of time, after which eye strain will make further viewing intolerable.
The reason for these problems is the lack of geometry common to both images. When the two images of a stereo pair are taken, the orientations of the camera during the scanning or framing process are almost unconstrained with respect to each other and instead are constrained with respect to the target. Certain scanning or framing parameters may be restricted for a stereo pair, but this is generally not enough to allow stereo perception of the raw data images. The situation is even worse for common coverage pairs, for which relative camera orientations are completely unrelated.
Presently existing solutions for the image registration problem have not created the capability to perform image registration autonomously. In present systems, an operator must extract conjugate points by hand. In the hard copy (film) world, this involves the use of a comparator that typically requires five to twenty minutes per stereo pair. However, these results are usually inaccurate and the images will not stay registered everywhere, but will typically drift out of registration during normal translational viewing of the image. The effect may be compensated for by manual adjustment, but adjustments are based on the perception of the stereo effect, rather than accurate measurement and results in degradation of the mensuration accuracy.
In the soft copy (computer data images) world, total registration is possible if a dewarping capability is available. Stereo dewarping takes the natural approach to image registration, by resampling the images to a common scanner or framing geometry. Theoretically, this method should present the images on a computer screen in a way that satisfies human requirements for stereo perception, based upon calculations using sensor flight and altitude data. In practice, however, stereo dewarping usually fails to register the images, because sensor flight and altitude data contains significant pointing and scan rate errors. Pointing errors usually contribute the largest misregistration effect and appear as an offset between the two images. Scan rate errors contribute additional misregistration by causing the images to drift out of registration during translation after the offset has been removed.
Thus, a need has arisen for an image registration system that will automatically register a common coverage pair or stereo image pair for use with a variety of image registration applications.